Piezoelectric key

ABSTRACT

A piezoelectric key for actuation by an attentionally directed and dimensioned compressive force includes a piezoelectric transducer for controlling an electrical amplifier element of an electronic circuit. The body of the transducer includes a directionally and permanently polarized piezoelectric material having electrodes situated thereon for taking off an electrical voltage. The body, in each plane perpendicular to the direction of the compressive force, has an approximately constant cross section and a thickness dimension d which is small with respect to another dimension of the body, the body being polarized in the direction of its thickness d. The electrodes are applied to the surfaces of the body which pulls each other in the direction d and the body is arranged in the key in such a way that the dimension d is essentially aligned perpendicular to the direction of application of the compressive force. The body is polarized throughout its volume and a dimension l (corresponding to length or circumference) which is essentially perpendicular to the thickness and to the direction of application of the compressive force is dimensioned several times greater than the dimension d, and the body has a dimension h which is perpendicular with respect to the dimensions d and l and which is larger than approximately ten times the dimension d.

Lilli -369 United St; a

Kleinschmidt E, 5

[ 1 PIEZOELECTRIC KEY [75] Inventor: Peter Kleinschmidt, Munich,

Germany [73] Assignee: Siemens Aktiengesellschaft, Berlin & Munich,Germany [22] Filed: Mar. 18, 1974 [21] Appl. No.: 452,060

[30] Foreign Application Priority Data Mar. 22, 1973 Germany 2314420[52] US. Cl. 310/95; 200/181; 235/145; BIO/8.1; 310/82; 310/87; 310/94;310/96; 310/98; 331/65; 331/163; 340/365 A [51] Int. Cl. H01L 41/10 [58]Field of Search 310/83, 8.7, 9.5, 9.6, 310/94, 9.1, 8.1; 200/181, DlG.4; 179/90 K; 235/145; 340/365 A; 317/144; 331/65, 116, 163

[56] References Cited UNITED STATES PATENTS 3,045,491 7/1962 Hart 310/8]UX 3,274,828 9/1966 Pulvari.. 310/8] UX 3,541,849 ll/l970 Corbett3lO/8.7 X

Primary ExaminerMark O. Budd Attorney, Agent, or FirmHill, Gross,Simpson, Van Santen, Steadman, Chiara & Simpson [451 Aug. 12, 1975 [5 7]ABSTRACT A piezoelectric key for actuation by an attentionally directedand dimensioned compressive force includes a piezoelectric transducerfor controlling an electrical amplifier element of an electroniccircuit. The body of the transducer includes a directionally andpermanently polarized piezoelectric material having electrodes situatedthereon for taking off an electrical voltage. The body, in each planeperpendicular to the direction of the compressive force, has anapproximately constant cross section and a thickness dimension d whichis small with respect to another dimension of the body, the body beingpolarized in the direction of its thickness d. The electrodes areapplied to the surfaces of the body which pulls each other inthe'direction d and the body is arranged in the key in such a way thatthe dimension d is essentially aligned perpendicular to the direction ofapplication of the compressive force. The body is polarized throughoutits volume and a dimension 1 (corresponding to length or circumference)which is essentially perpendicular to the thickness and to the directionof application of the compressive force is dimensioned several timesgreater than the dimension d, and the body has a dimension h which isperpendicular with respect to the dimensions d and l and which is largerthan approximately ten times the dimension d.

41 Claims, 19Drawing Figures PATENTED AUG 1 2 I975 SHEET PATENTED AUG 121975 SHEET PATENTED AUG 1 2 I975 SHEET III i Ill v 1 F 1lallulll V 1SHEET PATENTEU AUG 1 21975 F i g .14

PATENTEU 1 1 1975 PIEZOELECTRIC KEY BACKGROUND OF THE INVENTION 1. Fieldof the Invention This invention relates to a piezoelectric key or pushbutton which is actuated by a compressive force of a determinedmagnitude and direction, and more particularly to such a key whichcomprises a piezoelectric transducer for controlling an electricamplifier element of an electronic circuit wherein the body of thetransducer includes a permanently polarized, piezoelectric materialprovided with electrodes for taking off the electrical voltage andcurrent, the body having an almost constant cross section in every planeperpendicular to the direction with which the compressive force in thekey acts upon the body and having a thickness dimension d which is smallwith respect to another dimension of the body, the body being polarizedin the direction of the dimension d and the electrodes being located onsurfaces of the body which are oppositely disposed with respect to eachother at a distance determined by the dimension d.

2 Description of the Prior Art A large number of keys for connecting ordisconnecting electrical circuits is known in the art. Also, keys orpush buttons have been made known which comprise piezoelectric ceramicbodies for the production of an electrical voltage. Keys of this kindhave the advantage, for respective cases of application, that they notonly close a circuit, but they themselves are able to produce thevoltage for producing the current in the circuit from the mechanicalcompressive force acting upon such keys.

Piezoelectric keys have been realized, in particular, by means ofapplication of bending strips, at least a part of such bending stripsconsisting ofa piezoceramic material. In order to actuate such a key,the force acting upon a bending strip must create a distinctlynoticeable bending motion of the strip.

A further embodiment of a piezoelectric key has become known from theGerman Offenlegungsschrift No. 2,064,654 in which a disc-shaped memberof piezoceramic material is employed. The compressive force provided forthe actuation of the key acts upon the surface of the disc in such a waythat the disc is pushed in the direction of its thickness. Here thecompressive force for actuation is provided over a distance which liesbelow the limit of perceptibility available to man.

It has been observed that piezoelectric keys are sensitive totemperature changes which might occur during a more or less shortduration, that is over substantially short intervals of time. It hasbeen noticed that these keys can respond to changes of the ambienttemperature in the same manner as they respond to intentionally appliedcompressive forces. This effect occurs in the case of a piezoelectricbending strip due to its construction only to a subordinate extent;however. particularly the construction of a key with bending stripsrequires relatively great expenditures and efforts and in addition, aconsiderable actuating path.

The omission of distance of an actuating path for a switching key is ofparticular interest where a dust proof and/or moistureproof assembly isimportant, or where protection with respect to the possibility of adeliberate damage by means of avoiding slit-like openings alongrelatively movable parts is to be avoided.

SUMMARY OF THE INVENTION It is an object of the invention to provide apiezoelectric key which is to be operated with a compressive force whichis essentially actuation path free and which, within practical andsensible limits relative to its manner of operation, is insensitive toexternally acting temperature changes.

The foregoing object is achieved with a key, such as initiallydescribed, which is characterized according to the invention in that thepiezoelectric body is arranged within the key in such a way that itsthickness dimension d is essentially directed perpendicularly to thedirection in which the intentional compressive force in the key actsupon the body, that the body is polarized along the dimension d within avolume (determined by the dimensions d h I), that a dimension 1 (lengthor circumference) is essentially perpendicular with respect to thethickness d and is perpendicular with respect to the direction in whichthe compressive force in the key acts upon the body and is dimensionedseveral times larger than the dimension d, and that the body has adimension l1 which is perpendicular with respect to the dimensions d andl and which is larger than approximately l0 times the dimension d.

The most suitable values for the dimension 1 are between 3 and 30 mm.for actuating the key with a finger according to the invention. Inparticular, in the case of a key constructed according to the inventionwhich is actuated mechanically, it is advisable to measure the dimension1 according to the equation.

n 'lg K/U wherein K in the equation is the value of the intentionallyapplied compressive force acting upon the surface I d of the body in thekey, g;,, is the piezoelectric voltage constant of the material of thebody, and U is the value of the responding electrical voltage of thesubsequent electronic circuit.

A safe pyrointerval, i.e. the ratio between the value of thepiezovoltage or the piezopower, respectively, oc curring duringactuation and the possible value of a pyrovoltage or a pyropower,respectively, is obtained if the dimension d is chosen to be smallerthan 0.5 mm. A lesser margin for the dimension d results from thetechnological possibilities for the production of thin piezoelectricceramic bodies.

The elctrodes for taking off the piezoelectric voltage, I

at which also the pyrovoltage according to the invention occurs whichis-reduced in a proportional minimum, are applied to the surfaces h l ofthe body.

Additional details of the piezoelectric voltage constant 831 Can betaken from the state of the art, e.g., Valvo-Manualz Piezooxides, 1971,in particular on Page 20. The direction 1 coincides with the height h,

. as stated above, and the direction 3 coincides with the thickness d,also as stated above.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features and advantagesof the invention, its organization, construction and operation will bebest understood from the following detailed description of preferredembodiments of the invention, taken in conjunction with the accompanyingdrawings, on which:

FIG. 1 is a perspective view of a piezoelectric transducer mounted on asupporting surface showing relevant directions, dimensions andapplications of forces to aid in better understanding the presentinvention;

FIG. 2 is an elevational view of the apparatus of FIG. 2 showing moredetail of the structure of the piezoelectric transducer;

FIG. 3 illustrates the provision of a tubular body as a piezoelectrictransducer;

FIG. 4 illustrates the provision of an Sshaped body as a piezoelectrictransducer;

FIG. 5 illustrates potting of a piezoelectric transducer, such asillustrated in FIG. 3, for protection against moisture and for heatinsulation;

FIG. 6 is a perspective view of a particularly preferred embodiment fora piezoelectric key according to the invention;

FIG. 7 is an exploded view of an embodiment of the invention in whichthe transducer is located within a supporting member having an X-shapedprofile;

FIG. 8 is an exploded view of another embodiment of the invention inwhich supporting members have a high elastic resilience in the directionof the intentionally applied compressive force;

FIG. 9 is a perspective sectional view ofa particularly preferredembodiment of the invention for the construction of a piezoelectric keyin a mechanically rigid housing;

FIG. 10 illustrates the mounting of a tubular piezoelectric transducerwithin a heat insulating housing;

FIGS. 11-15 illustrate advantageous embodiments of piezoelectric deviceswith different electrode configurations;

FIGS. 16 and 17 illustrate preferred electronic circuits for theoperation of a key according to the invention; and E FIGS. 18 and 19illustrate a particularly preferred embodiment ofa key according to theinvention, shown in an exploded fashion, FIG. 19 illustrating theapparatus in section.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 will serve as anexplanation of the details of the teaching according to the presentinvention. In FIG. 1 a piezoelectric body 1, a ceramic body, is providedas an electromechanical transducer of a piezoelectric key and comprises,at least in a sub-volume 1 h d, which will be described in greaterdetail below, a material which is permanently polarized. Some of theknown materials which may be used to advantage for the body 1 are, forexample, barium-titanate and leadtitantezirconate, if necessary withadditives which improve the piezoelectric characteristics of thematerial.

As can be seen from FIG. I, the body 1 has a form with the dimension d,land the dimension [1 for this subvolume. The compressive force actingupon the body 1 in a predetermined manner or in the vectorial directionof an intentionally applied force is identified by the arrow 2. As longas there is no directional deflection and/or translation of thecompressive force affecting the key from the outside, the force 2 in thekey corresponds, according to direction and magnitude, to the forcewhich will be hereinafter referred to as an intentional" force or anintentionally applied" force which acts upon the body.

According to a feature of the invention illustrated in FIG. 1, thecompressive force in the key acts upon the surface 3 of the body in adirection perpendicular to the dimension d and parallel to the dimension/1 or H, respectively. The surface 3 of the body I which faces upwardlyin FIG. I has the lateral dimensions d and h. In order to simplifymatters, it is assumed that the compressive force 2 acts upon the centerof the surface 3. The compressive force, which necessarily opposes thebody with respect to the compressive force 2, is applied by a base plate4, which supports the body 1 and, as illustrated in a sectional view,supports the body 1 on the lateral surface which opposes the surface 3.

The body 1 is polarized in the direction d within the sub-volume 1 h d,as is indicated by the arrows 23. The electrodes 24 and 25 (onlyelectrode 25 being visible in FIG. 1, see FIG. 2) are located on thesurfaces 1 h of the body which are perpendicular with respect to thedirection of polarization, which electrodes 24 and 25 also serve for theapplication of a high electrical voltage for polarizing the body, aswell as serving for output electrodes for taking off thepiezoelectrically produced voltages for the subsequent electroniccircuit.

As already mentioned above, the sub-volume which is identified by theheight h is the part of the body 1 which acts piezoelectrically and,therewith, also pyroelectrically. It is advantageous to make the heightit somewhat smaller than the total height H of the body 1. A furtheradvantage, which will be described later on, resides in the provision ofheat insulation. A still further advantage resides in the details ofcircuit organization of the electronic circuit utilized with thepiezoelectric key.

The length l of the sub-volume is normally equal to the correspondingouter dimension L of the body itself. In order to avoid electrical shortcircuits between the electrodes 24 and 24 which are oppositely disposedwith respect to each other, it can be provided that the length l issomewhat smaller than the corresponding outer dimension, which is alsoreadily apparent from the structure illustrated in FIG. 1. As far as thedimension 1 of the polarized sub-volume is smaller than the outerdimension L of the body, K will be that part of the force which isapplicable to the dimension 1. However, this correction is not normallynecessary.

A particular embodiment or further development, respectively, of a keyconstructed according to the invention is carried out in such a way thatthe piezoelectric body of the key performs no, or practically only aninsufficient, bending motion under the influence of an intentionallyapplied compressive force. In a lateral view of the body 1 lookingtoward the surface H d in FIG. 2, a bending motion according to such anembodiment or development, respectively, is clearly indicated. The force2 applied to the surface 3 can lead to a deflection or bending of thebody 1, as is indicated by the broken lines 21. Such a deflection of thebody 1 results in piezoelectrically produced voltages in thepiezoelectric zone of the body 1, which voltages result in an overallvoltage between the electrodes 24 and 25 whose magnitude and polarity isdependent upon the type and direction of the deflection and whichpractically cannot be determined in advance. However, this voltage wouldbe superimposed with respect to the piezoelectric voltage intentionallyproduced by means of the longitudinal change of the length of the body Idue to the compressive force, and it would disturb or interfere withthis piezoelectric voltage in a manner which. for all practicalpurposes. cannot be determined in advance, provided a deflection, suchas mentioned above,

occurs.

Therefore, it is the aim of the specific embodiment or development,respectively, of the invention to eliminate the occurrence of adeflection of the piezoelectric body within the key, according to theinvention, at least so far that no disturbance occurs.

A bending motion can occur, aside from the greater force 2, by means offorges which deviate from the direction of the force 2. The references5, and 6, 16, respectively, designate compressive forces which areillustrated with broken lines and 'which, in practice, may possibly actupon the body 1. These compressive forces are assumed to be forcesacting perpendicularly to the compressive force 2 upon the upper part ofthe body 1 in FIG. 1. The forces 5, 15 and 6, 16, respectively, can beforce components of a compressive force acting upon the key, as assumedin practice, which is illustrated in FIG. 1 and identified with thereference characters 7. The component of the third direction in thecompressive force 2 acting upon the body 1 as an intentionally appliedforce.

The compressive force 7 can be, for example, the compressive forceexerted by a finger and actually externally acting upon the key.

It is guaranteed by the measures utilized for bending resistance, whichwill be explained in greater detail below, that in the body 1,practically only a compressive force understood to be an intentionalforce acts in the direction 2, in parallel to the dimension h, in alongitudinal change A h, and that an overly great compressive forceand/or force components in a direction perpendicular thereto, such asindicated by the references 5, l5 and 6, 16, respectively, are largelyineffective. The insensitivity of the key with respect to undesiredforce influences can still be increased according to furtherdevelopments, particularly by means of the mounting of the body into thehousing, as will be explained in greater detail below.

An embodiment of the invention in which the body is provided with aprofile having a bending resistance, is illustrated in FIG. 3. The bodyillustrated in FIG. 3 is in the form ofa tube. The form of a tube isadvantageous, whereby the cross section of the tube may be circular,oval or rectangular. It is important herein that the bending resistanceis present for a compressive force of such a direction in which acompressive force can be applied to the body in the key. At least in thenormal case, it will be sufficient that the bending resistance existsfor a compressive force acting in the direction of the axis of the tube,i.e. in the direction of the height H, upon the front surface 33.

In the case of application of a tube (instead of the body 1) as isdesignated 31 in FIG. 3, the average size of the circumference of thetube 31 is utilized as the dimension I according to the invention. Thewall thickness of the tube is to be taken for the thickness d of thebody. I

The tube in FIG. 3 has a pair of electrodes 34 and 35 applied to theouter and inner surfaces thereof, respectively, to serve for taking offof the piezoelectrically produced voltage. These electrodes extend overa distance corresponding to the height I1 and are illustrated with theborders thereof in broken lines. Referring for a moment to FIG. 5, thetube 31 is illustrated as being embedded in a casting resin forprotection against moisture and for heat insulation.

Comparable with a tube. a corrugated, curved or wavy profile, such asthe S-shaped structure illustrated in FIG. 4 may be utilized as thepiezoelectric body, referenced 41 in FIG. 4. The length [of the profilecorresponds to the dimension 1, as is marked on the drawing. The body 41is piezoelectrically characterized across its entire length 1 and isprovided with electrodes 44 and 45 which are illustrated within thebroken lines.

The force 2 acts upon the surface 43, which correponds to the surface 3of the body 1 in FIGS. 1 and 2.

The utilization of a supporting member serves, according to the alreadyspecified further development, for avoiding a bending motion of apiezoelectric body, which supporting member provides thepiezoelectrically effective body with the required bending resistance bymeans of its shape and by means of its association with thepiezoelectrically body, if need be, in cooperation with the latter. Thematerial and the dimensions of the supporting member are selected insuch a way that the supporting member of the predetermined compressiveforce which is effective in the key, has, in comparison to the body ofthe transducer, a greater or not considerably smaller integralpressure-elastic resilience.

An embodiment of this development with a supporting member provides thatthe body of the transducer, e.g., the body 1 or 31, is embedded in anelastic casting resin, whereby only the surface 3 or 33, respectively,and the surface which is oppositely disposed with respect to suchsurface, stands on the supporting base 4 are essentially free from thecasting resin. FIG. 5 illus trates such an embedding for a small tube 31as illustrated in FIG. 3, in an exploded view. The casting resin isdesignated with the reference character 50 and a cover plate isidentified with the reference character 52. The cover plate 52 receivesthe intentional compressive force and transmits the same to the smalltube 31.

FIG. 6 schematically illustrates a particularly preferred embodiment fora key, according to the invention, wherein a pair of small plates 64 and164 are applied to the body 1 of the transducer as supporting members onthe two large surface sides I H. The small plates are preferably bondedto the body 1. A thin coating consisting of thermosetting plastic, e.g.,Technicoll 401 or 411, respectively, with subsequent heat treatment, hasproven to be suitable as an adhesive material. The body 1 and the smallplates 64 and 164 therefore form a body unit 101 which is resistant tobending due to its layer construction. In the direction of thecompressive force 2 which acts upon the surface 3 of the body 1 and uponthe surfaces 63 and 163 of the small plates 64 and 164, the plates 64and 164 are much more resilient than the ceramic of the body 1.Accordingly, the compressive force 2 is primarily effective in thebody 1. A polyacrylic glass has proven to be suitable as a material forthe small plates 64 and 164. The thickness of the plates is selectedsuch that the necessary bending resistance is obtained. It should betaken into account that, when bonding the plates to the body 1, thebending resistance-apart from the differences of the modulus ofelasticityincreases with the third power of the sum of the individualthicknesses, and the resistance with respect to the compressive force 2increases only linearly with the total thickness.

The electrodes 24, 25 and 124, 125 in FIGS. 1 and 2 are situated on thebody 1 and sandwiched between the respective surfaces of the body 1 andthe small plates 64 and 164.

FIG. 7 illustrates, in an exploded view. an embodiment of the inventionin which the body 1 of the transducer is located within a supportingmember 70 having an X-shaped profile. A closing part 74, correspondingto the base plate 4, yields little with respect to the material of thebody 1. A cover plate 72, which corresponds to the cover 52 in FIG. 5,covers the upper surface and carries a plate 76, consisting of amaterial having a small thermal coefficient of expansion, e.g. Invar.

FIG. 8 illustrates a further embodiment of the invention, which is shownexploded in both the longitudinal and transverse directions, and whichcomprises a transducer body 1 and a pair of supporting members 83 and 85constructed out of metal having a low thermal expansion coefficient,e.g., Invar.

The supporting members 83 and 85 are bent as illustrated in the drawingand includes slot-like openings to provide the supporting member with ahigh elastic resilience in the direction of the compressive force 2. Thetransducer body 1 and the supporting members 83 and 85 are arrangedagainst each other and between and against the adjacent pressureresistance plates 84 and FIG. 9 illustrates a particularly preferredembodiment for the construction of a key according to the invention in across section perspective view. In FIG. 9 the key includes amechanically rigid housing 91. The body 1, with the supporting members64 and 164, i.e., the unit 101 of FIG. 6, is situated between the coverplate 92 which receives the intentionally applied force and the baseplate 94 which is necessary for providing the counter pressure, the baseplate 94 corresponding to the plate 4 in FIG. I. The transducer bodywith the supporting members is held in a groove 95 formed in the baseplate 94. On the inner side of the plate 92, the transducer body withthe supporting members is secured with respect to lateral displacementin a holding block 96 which is provided with a slot comparable to theslot 95 in the base plate 94. In comparison to the dimension 1, theholding block 96 is short, which is readily apparent from the drawing,whereby such forces can be rendered inoperative with respect to the bodyunit 101, which forces would otherwise be able to act transversely uponthe body unit due to thermal expansion of the plate 92.

Due to a somewhat flexible mounting in the slots, the transducer bodywith its supporting members is insensitive to obliquely actingcompressive forces, such as the force 7 in FIG, 1, since transversecomponents of the force 7 cannot cause a bending of the transducer body,even when the housing 90 is somewhat resilient.

In the exemplary embodiment illustrated in FIG. 9, the particularypreferred further development, which is also illustrated in FIG. 1, isrealized at the transducer body, according to which development theupper part of the transducer body 1 in the housing (above the height 11)is piezoelectrically inactive. This part is also pyroelectricallyinactive. Accordingly, the key of FIG.

moderate temperature changes of the plate 92 are encountered in order toprevent pyrovoltages at the body 1. The compressive force, which istransmitted by the relatively narrow holding block 96, is distributed inthe inactive upper sub-volume onto the entire length l of the activepart 11.

In the case of the embodiment according to FIG. 9, the plate 92, uponwhich the pressure acts, has little bending resistance and transmits theessential part of the intentional compressive force to the body unit 101or the body 1. The lateral parts of the housing are relatively slightlyresilient in the longitudinal direction. These lateral parts areprovided with projecting bosses 98 and 99 which serve for mounting ofthe key.

Connection leads 224 and 225, illustrated in FIGS. 1 and 9, are providedfor the electrodes 24 and 25 on the body 1 and lie beneath the supportedplates 64 and The body 1 of the exemplary embodiments of FIGS. 6 and 9consisting of a single piece and having a piezoelectrically effectivepart 11 and a non-piezoelectric part H 11 can also consist of two piecesh and H h which are disposed one on top of the other, as if they were asingle piece as illustrated in FIG. 1. The two pieces are advantageouslyheld together in this position by the small plates 64 and 164.

FIG. 10 illustrates an embodiment of the invention in which thecompressive force 200, which acts intentionally upon the body of thetransducer in the key, has a direction which deviates from theintentional compressive force 2 which acts upon the key itself from theoutside.

A small tube 31, such as illustrated in FIG. 3, is provided as thepiezoelectric body. Preferably, the housing is heat insulating and, asillustrated in a sectional view, is designated with the referencecharacter 100. A plate, which is identified with the reference character102, is provided to be deflected by the intentionally appliedcompressive force 2. As a result of the deflection of the plate 102, theforce and counter force 200 act upon the ends of the small tube 31.

The invention is based on consideration which will be described asfollows. With the help of the piezoelectric effect, it is possible, whenutilizing a permanently aligned polarized, i.e. piezoelectric, ceramicto produce an electrical voltage by means of the application of acompressive load, which voltage is suited for the control of anelectronic circuit. An electromagnetic relay switch can be actuated whenproceeding from this circuit. However, as it was found in earlierdevelop ment tests, a great pyro-effect occurs in the case of apiezoelectric ceramic, due to the great temperature dependency of thepolarization, which pyro-effect, as mentioned above, can lead toelectrical voltages which may easily be of the same magnitude aspiezoelectrically produced voltages, or which may even be greater bysome orders of magnitude. Therefore, a conductive discharge always hasto be provided in the case of the piezoelectric key, whereby thedischarge may occur via the conductivity of the ceramic itself and/0rvia a parallel resistance. It is possible, though, by means ofparticularly costly heat insulation to influence the pyroeffect, atleast to such an extent that the load displacements, on which thepyrovoltage is based, takes place at such a slow pace that they aresmall per unit of time in comparison to the load displacements which areproduced piezoelectrically due to the influence of pressure, which meansthat the pyrocurrent, i.e., the low displacement per unit of time whichis subject to heat, is small with respect to the piezocurrent.

ln the case of the key according to the invention, uses made of thistechnical measure for the reduction of consequences of the pyroeffect,which will be explained in detail in the following exemplaryembodiments. It is an important fact of the invention that theindependence of temperature of the key is based on constructivemeasures, namely the dimension and the arrangement of the body of thetransducer in the key which is dimensioned in such a way that thevoltages or currents, respectively, which are piezoelectrically producedby the intentional compressive force are considerably above the valuesof pyrovoltages or pyrocurrents, respectively, which may occur inpractice as a result of temperature changes of the transducer bodyappearing over an interval of time whereby the time intereval is themaximum duration occurring in practice for the considerable increase ofthe compressive force.

By means of the teaching of the present invention, a considerable safetyfactor is achieved in the key for the piezoelectric useful voltageproduced by the intentional compressive force with respect to thepyroelectric voltage, as well as for the corresponding currents. Thethresholds of the subsequent electronic circuit whichp processes thepiezoelectric voltage signal and the current signal can therefore have aconsiderable difference from the maximum possible pyrovoltage, and thekey, according to the invention, together with the circuit will stillindicate a signal, even if a possibly somewhat slighter pressure isapplied.

According to the invention, an important feature of the transducer bodyis that the compressive force acting upon the body stresses the bodyedgewise, that is longitudinally. This means that the thicknessdimension d of the body which is transverse to the direction of pressureis small, at least three times smaller, preferably at least ten timessmaller, than the directionally measured height h of the body. Thethickness d is normally 0.5 mm. The pyrovoltage increases as thethickness increases, but the pyrovoltage does not change with constantcompressive force. lt is advantageous to make the body considerablythinner than 0.5 mm, since a greater difference between the piezovoltageand pyrovoltage is achieved with such thinner bodies. Primarily,technological difficulties oppose a decrease in thickness.

The dimension 1, together with the height h is impor tant for thesurface magnitude of the electrode and therewith for the magnitude ofthe piezocurrent supplied by the key when pressure is applied. A minimumcurrent is to be produced during pressure application so that the inputtransistor of the subsequent electronic circuit can be controlledaccordingly. It is advisable to dimension the value 1 with )1 between0.3 and 2.0, according to the formula )1 I831! K/U for pressure valueswhich are exerted with the finger and which amount to approximately 1Newton, values between 3 and mm can be obtained in the case of thresholdvoltages of about 1 volt. In the case of such values for l and togetherwith values for h, which will be discussed later on, the key, accordingto the invention. suppliesin the case of finger pressure with a pressurechanging speed of about 10 Newton/seconds-piezocurrent intensities whichare entirely sufficient to control a bipolar transistor. ln other words.a key is obtained with a low impedance which is suited for the directcontrol of bipolar transistors. Surprisingly, 3 to 20 mm for the value 1result in a dimension which is practically not greater than the width ofthe finger so that the force supplied by the finger can actually beexerted onto the entire surface (surfaces 3 in FIGS. 1 and 6 and 33 inFIG. 3). From this it follows that for the pyrodistance the crosssection 1 d is to be made small within the frame work of technicallyrealizable dimensions, whereby in the case of the above stated valuesfor l, a value proves to be suitable for the thickness d which is smallin comparison to the value of l.

A value of a few millimeters, particulary between 5 and 25 mm, resultsfrom the above stated proposals for dimensions with respect to theheight.

For the control of a bipolar transistor with a given threshold voltageof 0.6 volt and a finger pressure of 1 to 2 Newton, the followingdimensions are preferred for a body in an embodiment according to FlG.6:

d= 0.5 to 0.1, or rather 0.15 mm.

1 5.0 to 10 mm.

h 5.0 to 20 mm.

For the small plates 64 and 164 which are provided as supportingmembers, a copolymer of vinylestervinylchloride (Astralon), apolyacrylnitrile (Plexiglass) or polystyrol having a thickness of 0.3 to0.6 mm has been selected for each of the plates. As opposed to thesematerials, the'ceramic has a modulus of elasticity which isapproximately 20 times greater.

A value of 10' V-m/Newton can be taken as a basis for the magnitude ofthe piezoelectric constant g Whe actuating a key thus dimensionedaccording the invention, a piezoelectrically produced power of about 10W with a source capacitance of 15 nF was achieved in the case of anincrease force of 1 Newton, as is typical for finger pressure, during anactuating period of 0.1 second. The deformation of the body of thetransducer is approximately 0.3 um in the case of a dimension as statedwith the force of 1 Newton. A key constructed according to the inventionhas the advantage of being able to operate without an idle current whichis necessary, for example, in the case ofa capacitive key which is alsopath less.

Basically, a sufficiently great difference between the pyrovoltageoccurring at normal temperature changes and the useful piezovoltageproduced by the intentional pressure is already achieved in the normalcase by the dimensions of the body of the transducer according to theinvention, in particular by a small thickness or wall thickness,respectively, in regard to the other dimensions, preferably in regard tothe height d of the polarized sub-volume. A supporting member which isprovided, if required, has an advantageous influence because it effectsa certain heat insulation of the piezoelectrically and therewith alsopyroelectrically effective body of the transducer. Such heat insulatingsupporting members are, for example, the small plates 64 and 164, thesealing compound or the supporting member 70.

A further enlargement of the pyrodistance, i.e., 0f the relation betweenuseful piezovoltage and undesired pyrovoltage, can be achieved by meansof two further measures which will be described in detail in thefollowing paragraph.

The total height of the body 1 of the transducer is designed with thedimension H in FIGS. 1 and 2. However, the transducer of this examplaryembodiment is polarized only up to the height h and is therewithpiezoelectric only up to this height, as was described in the foregoingdiscussion. A piezoelectrically produced voltage can be collectedbetween the electrodes 24 and 25 when pressure is applied to the body 1.However, in the zone H h of the body 1, i.e., between the electrodes 124and 125, no piezoelectric voltage occurs due to the lack of an alignedpolarization of the material of the body, not even when pressure isapplied. By the same token. however, a pyroelectric voltage does notdevelop due to this lack of an aligned polarization.

Because the body 1 is divided into a piezoelectrically (andpyroelectrically) effective and a piezoelectrically (andpyroelectrically) ineffective part, a very good heat insulation of theeffective partial volume h l d can be effected with respect to heat flowfrom the surface 3. As is illustrated in FIG. 1, the surface 3 is theupper front surface of the nonpolarized part of the body 1.

The aforementioned division actually leads to a loss of mechanical workwhich is required for the actuation of the key according to theinvention. This loss consists in that the nonpiezoelectrically effectivesub-volume of the body 1 also experiences a mechanical deformationbetween the electrodes 124 and 125 which, however, does not supply apiezovoltate. In the normal case, such a loss of work is insufficientsince the increase, which is necessarily connected therewith, of therequired path of the determined compressive force is of no interestbecause the total occurring actuating path is already inperspectivelysmall.

The relation of the height h to the total height H is 9 preferablyselected between 0.8 and 0.6. The increase in distance of the actuatingpath which is already inperspectively small in the normal case is about20% to 40% in the case of this dimension. The above described particularfurther development of the body 1 is also advantageous for other formsof piezoelectric transducer bodies of the key constructed in accordancewith the invention. In particular, in the case of the tube shaped member31 illustrated in FIG. 3 and in the case of a number 41 having a curvedprofile, as illustrated in FIG. 4, a division of the body can beprovided with respect to the direction of an intentionally appliedcompressive force.

Another technique for providing thermal insulation of a key according tothe invention includes the mounting of the body of the transducer of thekey into a heat insulating housing. This measure may be provided inaddition to the measures for heat insulation already described.

FIGS. 5, 7, 9 and 10 illustrate mountings having a thermal insulatingeffect. It is particularly important in the case of heat insulation, andthe same applies also to heat insulation by means of the describedsupporting members, that the body of the transducer be insulated withrespect to rapid temperature changes. A low temperature change of thebody in a key constructed according to the invention, results in onlysuch pyrovoltages which increase as slowly and therewith considerablyslower than the piezoelectric useful voltage. By means of an electricaldischarge with a high pass effect, e.g. in the form of an electricalresistance connected in parallel with respect to the piezoceramic, theconstruction ofa free load which causes the pyrovoltage can besuppressed.

FIGS. 11-15 illustrate advantageous configurations of the electrodesapplied to the piezoceramic body.

FIG. 11 illustrates a piezoelectric body having a height [1 in a frontalview and FIG. 12 is a lateral view of the same structure. A continuouselectrode 1024 is provided on one side of the body 1. On the oppositeside of the body 1, the electrode 25 according to FIG. 1 is separatedinto two individual electrodes 1025 and 1026. The polarization of thematerial of the body 1 below the electrodes 1025 and 1026 is indicatedby the arrows 1021 and 1022. The polarization among these individualelectrodes is directed in opposite directions with respect to eachother. Connection lines, electrical leads, are identified with thereference characters 1224 and 1225. Due to the polarization in oppositedirections, it is sufficient to contact the electrodes on one side ofthe body 1 with the indicated connections. The upper and the lowerhalves of the body 1 are electrically connected in series via theelectrode 1024.

FIG. 13 illustrates an embodiment of electrodes corresponding to thatillustrated in FIGS. 11 and 12 in which the individual electrodes 1025and 1026 shown in FIGS. 11 and 12 are arranged next to one anotheracross the dimension land extending in the direction of the dimension11.

FIG. 14 illustrates an electrode arrangement corresponding to theembodiment and according to FIGS. 11 and 12 for a tube shaped body ofthe transducer according to FIG. 3. A continuous electrode 1135 isprovided on the inner surface of the small tube 31. The electrode 34according to FIG. 3 is divided on the outer surface of the tube 31 intotwo individual electrodes 1134 and 1234 which are arranged next to eachother in a ring-like manner. In the zone of the ring-shaped electrode1134, the material of the small tube 31 is oppositely polarized withrespect to the zone adjacent the electrode 1234.

FIG. 15 illustrates an embodiment in which the outer electrode 34according to FIG. 3 is divided into two peripheral halves as individualelectrodes having the designations 1334 and 1434. These two singleelectrodes are separated from each other on the reverse side of the tube31 which is not visible in the drawing. The material of the tube 31 inthe zone of the first single electrode is oppositely polarized withrespect to the material in the zone of the other single electrode.

Also, in the case of the embodiments according to FIGS. 1315, it issufficient to contact electrodes on only one side of the body 1 or 31,respectively, of the transducer. The respective counter electrodes 1024,I035 continue over the individual electrode of the opposite side. Thezones adjacent the individual electrodes are therefore connected inseries by way of the counter electrode.

A particularly preferred electronic circuit for the operation of a keyaccording to the invention is illustrated in FIGS. 16 and 17 and will bediscussed in detail below.

Referring to FIGS. 16 and 17, the piezoelectric body of atransducer isdesignated 201 in FIG. 16. Upon the application of an intentional forceupon the transducer 201, a piezoelectric voltage is produced at theconnection points 203 and 205 or a piezoelectric current, respectively,can be collected at these points by way of the electrical leads 224 and225. A bipolar input transistor 207 is provided for responding to theoutput of the transducer. In the case of an npn transistor, the materialof the transducer body 201 is polarized in such a way that the terminal203 is provided with a positive potential with respect to the terminal205 upon the application of an intentional force. An electrical resistor209 is connected between the base and the emitter of the transistor 207.The resistor 209 provides a shunt discharge path for charges at theelectrodes 24. of the body of the transducer which develop during longperiods. Such long term charges appear, in particular. by means oftemperature changes of the material of the body of the transducer as aresult to the aforementioned pyroeffect. The resistor 209 has aresistance value in the order of 10 ohms. The resistor 209 and thecapacitance of the piezoelectric portion of the body 201 advantageouslyform a high pass filter, the resistance and capacitance values of whichare dimensioned at a cut off frequency of s 10 Hz. For a key constructedin accordance with the invention, the correct values for obtaining thishigh pass filter are easily obtainable.

An RC circuit is advantageously provided by a resistor 211 and acapacitor 213. This RC circuit has the purpose of rendering the keyparticularly insensitive to vibrations. Very high frequency,piezoelectrically occurring voltage pulses might appear due to verygreat vibrations of the body of the transducer. These pulses have aconsiderably higher frequency than the piezoelectric useful voltagepulses caused by an intentional compressive force. Resistance andcapacitance values of the RC circuit 211, 213 are selected in such a waythat the useful voltage pulses are not, or only insufficiently passedand the interference pulses are practically short circuited. Values ofabout 10 Hz can be taken as a basis as frequency values for the usefulvoltage pulses and values of l KHZ are more for the interference pulses.

It is advisable to connect a further transistor stage to the transistor207. With such a stage a switching current controlled by the key may beobtained in the order of magnitude of 10' ampere, at the outputconnections 219 and 221. This magnitude of current is suffi cient foractuating relays or the like which are symbolically illustrated by theresistor 231.

With the two stage circuit illustrated in FIG. 16, the very high ohmicresistor 209 can be replaced by a resistor 233, showing broken linesconnected between the base and the emitter of the transistor 213, whichresistor has a resistance value which is smaller by several orders ofmagnitude. The reduction factor is equal to the factor of the currentamplification of the transistor 207. A diode 235 is connected betweenthe base and the emitter of the input transistor 207 so that, in thecase of such a circuit with a resistor 233 instead of the resistor 209,no pyrocharging voltage of opposite polarity can be effective from theterminals 203 and 205.

As has been already indicated above, the part H h I of the body I of thetransducer, i.'e. the nonpiezoelectrically effective part in the zonebetween the electrodes 124 and 125, can very advantageously be utilizedfor the capacitor 213. FIG. 16 illustrates a corresponding embodimentfor the circuit according to FIG. 11. Details already mentioned abovehave corresponding reference characters in FIG. 17. The electricalconnections for the electrodes 124 and 125 have been provided with thedesignations 324 and 325.

The resistance associated with the RC circuit consisting of the resistor211 and the capacitor 213 can be applied on one surface of the body ofthe transducer, in particular in the case of a flat embodiment of atransducer, as illustrated in the drawings. This resistance may even bea portion of one of the electrodes on the body. in particular a part ofthe electrode 24 or 25.

In accordance with the features of a key constructed according to theinvention, in particular in accordance with the selection of aligningthe polarization perpendicular to the direction of an intentionalcompressive force, the so-called piezoelectric transverse effect isexploitedv An advantageous impedance adaption of the piezoelectric bodyof the transducer of the key to the given input impedance of theelectronic transistor circuit of the key can therefore be achieved.

The electronic circuit of the key is preferably also mounted into thehousing of the key, i.e., within the housing 90.

A particularly preferred method for the production of a key providedwith supporting members will also be described hereinbelow. This methodrelates. in particular, to a key with a flat body 1 for the transducer,and deals with the mechanical connection between the body 1 of thetransducer and one or several supporting members, e.g., the supportingmembers 64 and 164 in FIG. 6. This particularly preferred methodcomprises the step, while using particularly duroplastic materials(thermosetting plastic materials) of connecting one or severalsupporting members mechanically rigid with the body 1 of the transducer.The duroplastic material may be a coating consisting of such a materialprovided on the corresponding side of the supporting member, or a layerconsisting of duroplastic material may be inserted. The one or severalsupporting members of the body of the transducer are pressed together bymeans of application of pressure and heat so that the duroplasticmaterial is able to create a durable mechanical connection. Ifnecessary, corresponding bores, windows, apertures, recesses or the likemay be provided in the supporting members for electro connections to theelectrodes 24, 25 or 124, 125, respectively. Electrical connection meansmay be provided in the recesses which create the contact to theconnection contact provided at the body. The electrical connectionbetween the connection contacts and the contacts in the recesses canadvantageously be realized by known thin layer conductor paths.

Referring to FIGS. 18 and 19, a particularly preferred embodiment ofakey according to the invention is illustrated as comprising a resistor211 and a capacitor 213, constructed in accordance with the above methodand referenced with different characters to better illustrate thestructural formation thereof. In FIGS. 18 and 19 essential parts of thekey shown which were illustrated in FIG. 6, but which here have beengiven some additional identifying reference characters, namely thepiezoelectric body 1 of the transducer has the small supporting platesreferenced 864 and 8164. These supporting members correspond to thesupporting members 64 and 164 but have been provided with projectingfeet 801 and 8101 and bores 802 and 8102 as recesses for providingcontacts. The illustration in FIG. 18 can also be referred to as anexploded illustration in that these elements. in practice, arecontacting in a layer type arrangement. FIG. 19 is, of course, a crosssection of the exploded apparatus in FIG. 18. taken generally along theline XIX-XIX.

The electrodes of the body 1 are identified as 824 and 825. Theelectrode of the capacitor 213 carries the designation 8125 and can becompared with the electrode 125 of FIG. 1. The counter electrode to theelectrode 8125 is the upper part of the electrode 824. The electrodes825 and 8125 have extending feet arranged on the corresponding feet 801and 8101 of the supporting members 864 and 8164. Between these extendingconductors, here referenced 8224' and 8225' is arranged a resistor 8211.eg, a resistance layer. which is applied to the body 1. This resistorhas an electrical contact with the electrode 8125 at its one end andwith the extending projection 8025 of the electrode 825 at its otherend. The resistor 8211 is a realization of the aforementioned resistor211, and its resistance value is dimensioned corresponding to theresistor 211 by the selection of width and thickness as well as by theapplied resistance material of the element 8211.

Electrical connections of the electrodes 824 and 8125 are realized bymeans of the thin layer electrical connections 8224 and 8225, each ofwhich extends through a respective bore 802 and 8102. Instead ofconnections extending through the bores, the bores may also be filled ina contacting manner with an electrically conductive material. Theaforementioned feet 801 and 8101 may serve, for example, for insertingthe piezoelectric transducer, which is mounted in a housing (notillustrated) together with the supporting members 864 and 8164 into asupport structure which is provided for the piezoelectric key, e.g.,P16. 9. During the process of inserting the transducer into a supportstructure, the electrical connections are effectedat the connectioncontacts 8224 and 8225' of the key, which electrical connections areidentified as 224 and 225 in the circuit according to FIG. 16.

It may be pointed out that the dimensions in FIGS. 18 and 19 inparticular the thickness dimensions of the body as well as of theelectrical coatings are illustrated in an exaggerated form so that theyappear considerably thicker in comparison to practical cases ofapplication. This distortion of scale is provided to guarantee a cleareridentification of the elements, actual dimensions for embodiments, whichalso pertain to FIGS. 18 and 19, have already been given here before.

Although I have described my invention by reference to a particularembodiments thereof, many changes and modifications of the invention maybecome apparent to those skilled in the art without departing from thespirit and scope of the invention. 1 therefore intend to include withinthe patent warranted hereon all such changes and modifications as mayreasonably and properly be included within the scope of my contributionto the art.

1 claim:

1. A key for controlling an amplifier of an electronic circuit inresponse to the application thereto of an intentionally applied anddimensioned compressive force, comprising: 1

a piezoelectric transducer including a body of piezoceramic material forreceiving the compressive force,

a pair of electrodes on said body for taking off electrical signals andfor connection to the amplifier.

said body having a substantially constant cross section perpendicular tothe compressive force,

said body polarized in a direction d in at least a subvolume (d l 11)thereof where d is the thickness dimension. 1 is the length dimension,and h is the height dimension, said electrodes mounted on oppositesurfaces of said body spaced apart the thickness d.

the thickness d perpendicular to the direction of the compressive force,the length 1 being perpendicular to the thickness (1 and to thedirection of the compressive force and is greater than the thickness (1.and the height h is perpendicular to the thickness d and the length land is greater than 10d,

2. The key of claim 1, wherein said body includes a base surface havingan area I d of less than 5 mm 3. The key of claim 2, wherein said basesurface has an area of 1 mm*.

4. The key of claim 1, wherein the length l is at least 20d.

5. The key of claim 1, wherein the length l is d.

6. The key of claim 1, wherein the length I is in a range of from 5 to20 mm.

7. The key of claim 1, wherein the length l is dimensioned according tothe equation l= n.|g K/U where K is the compressive force acting uponthe surface 1 a' of said body, g is the piezoelectric voltage constantof the material of said body, U is the response voltage necessary foractuating the electronic circuit and n is a value in a range of from 0.3to 2.0.

8. The key of claim 1, wherein the thickness 1 is less than 0.5 mm.

9. The key of claim. 1, wherein the thickness d is in a range of from0.05 to 0.15 mm.

10. The key of claim 1, wherein said body includes surface areas definedby the dimensions 1 h and said electrodes cover respective ones of suchsurface areas.

11. A key for controlling an amplifier of an electronic circuit inresponse to the application thereto of an intentionally applied anddimensioned compressive force, comprising:

a piezoelectric transducer including a body of piezoceramic material forreceiving the compressive force,

a pair of electrodes on said body for taking off electrical signals andfor connection to the amplifier, said body having a substantiallyconstant cross section perpendicular to the compressive force,

said body polarized in a direction d is at least a subvolume (d l 11)thereof where d is the thickness dimension, 1 is the length dimension,and I2 is the height dimension, said electrodes mounted on oppositesurfaces of said body spaced apart the thickness d,

the thickness d perpendicular to the direction of the compressive force.the length I being perpendicular to the thickness d and to the directionof the compressive force and is greater than the thickness d,

the height I1 is perpendicular to the thickness d and the length I andis greater than 10d, and

support means mounted against said body and cooperable therewith toresist bending in response to the application of the intentionallyapplied compressive force.

12. The key of claim 11, wherein said support means, in the direction ofthe force, has an integral pressure elastic resilience which issubstantially the same as to greater than that of said body.

13. The key of claim 11. wherein said support means comprises a pair ofplates fixed to opposite surfaces of said body.

14. The key of claim 11, wherein said support means comprises a memberembedding said body and having an X-shaped profile in the directionparallel to the direction of force application.

15. The key of claim 13. wherein each of said plates has dimensionscorresponding to the corresponding dimensions of the respective surfaceon which it is fixed.

16. The key of claim 15, wherein each of said plates has a thicknesswhich provides the bending resistance and an integral pressure elasticresilience greater than that of said body in the direction of theapplied force.

17. The key of claim 13, wherein said plates are mounted on the twolargest lateral surfaces of said body.

18. The key of claim 13, wherein said plates are bonded to said body.

19. The key of claim 13, wherein said plates consist of syntheticmaterial.

20. The key of claim 11, wherein said housing includes a forcetransmission member, mounting said body only in a small Zone of saidbody.

21. The key of claim 20, wherein said housing includes externalsurfaces, a force receiving member, and

mounting flanges extending from said surfaces adjacent said member.

22. The key of claim 11, comprising supporting means including a pair ofsupport plates, and thermoplastic bonding means securing said plates toopposite surfaces of said body.

23. The key of claim 22, wherein said plates include recesses forreceiving respective electrical connections for said electrodes.

24. The key of claim 23, comprising conductive strips carried in saidrecesses and connected to the electrodes.

25. A key for controlling an amplifier of an electronic circuit inresponse to the application thereto of an intentionally applied anddimensioned compressive force. comprising:

a piezoelectric transducer including a body of piezoceramic material forreceiving the compressive force, I

a pair of electrodes on said body for taking off electrical signals andfor connection to the amplifier,

said body having a substantially constant cross section perpendicular tothe compressive force,

said body polarized in a direction d in at least a subvolume ((11.11)thereof where d is the thickness dimension, I is the length dimension,and h is the height dimension,

said electrodes mounted on opposite surfaces of saidbody spaced apartthe thickness d,

the thickness d perpendicular to the direction of the compressive force,the length I being perpendicular to the thickness d and to the directionof the compressive force and is greater than the thickness d,

the height h is perpendicular to the thickness d and the length I and isgreater than d. and heat insulating means surrounding said body. 26. Thekey of claim 25, wherein said heat insulating means comprises a materialwhich is less compression resistant than said body.

27. A key for controlling an amplifier of an electronic circuit inresponse to the application thereto of an intentionally applied anddimensioned compressive force, comprising:

a piezoelectric transducer including a body of piezoceramic material forreceiving the compressive force,

a pair of electrodes on said body for taking off electrical signals andfor connection to the amplifier,

said body having a substantially constant cross section perpendicular tothe compressive force,

said body polarized in a direction d in at least a subvolume (d l 11)thereof where d is the thickness dimension, [is the length dimension,and h is the height dimension,

said electrodes mounted on opposite surfaces of said body spaced apartthe thickness d,

the thickness d perpendicular to the direction of the compressive force,the length 1 being perpendicular to the thickness d and to the directionof the compressive force and is greater than the thickness d,

the height h is perpendicular to the thickness d and the length l and isgreater than 10d, and

a housing for said transducer, said housing including a base, a cover toreceive the force, a body mounted between said base and said cover, andwalls adjoining said base and said cover and having less compressionresistance to the force than to said body.

28. The key of claim 27, wherein said housing includes a base having agroove therein and a cover having grooved means, said body mounted insaid groove and said grooved means.

29. A key for controlling an amplifier of an electronic circuit inresponse to the application thereto of an intentionally applied anddimensioned compressive force, comprising;

a piezoelectric transducer including a body of piezoceramic material forreceiving the compressive force,

a pair of electrodes on said body for taking off electrical signals andfor connection to the amplifier,

said body having a substantially constant cross section perpendicular tothe compressive force.

said body polarized in a direction din at least a subvolume ((1 I 11)thereof where a is the thickness dimension, 1 is the length dimension,and I1 is the height dimension,

said electrodes mounted on opposite surfaces of said body spaced apartthe thickness d,

the thickness d perpendicular to the direction of the compressive force,the length I being perpendicular to the thickness d and the length 1 andis greater than 10d, and

a housing for said transducer including a base and a cover platemounting said body therebetween, and a plurality of side walls joiningsaid base and said cover, said side walls being resilient with respectto the compressive force and said base being slightly resistant tobending.

30. A key for controlling an amplifier of an electronic circuit inresponse to the application thereto of an intentionally applied anddimensioned compressive force, comprising: 7

a piezoelectric transducer including a body of piezoceramic material forreceiving the compres sive force,

a pair of electrodes on said body for taking off electrical signals andfor connection to the amplifier, said body having a substantiallyconstant cross section perpendicular to the compressive force,

said body polarized in a direction 41 in at least a subvolume (11 I h)thereof where d is the thickness dimension, 1 is the length dimension,and I2 is the height dimension,

said electrodes mounted on opposite surfaces of said body spaced apartthe thickness d,

the thickness d perpendicular to the direction of the compressive force,the length I being perpendicular to the thickness d and to the directionof the compressive force and is greater than the thickness d,

the height 12 is perpendicular to the thickness d and the length I andis greater than d, and

a second electrode mounted adjacent and electrically insulated from andhaving the same surface as one of the first-mentioned electrodes, saidbody being polarized in opposite directions in the zones adjacent saidadjacent electrodes.

31. A key for controlling an amplifier of an electronic circuit inresponse to the application thereto of an intentionally applied anddimensioned compressive force, comprising:

a piezoelectric transducer including a body of piezoceramic material forreceiving the compressive force,

a pair of electrodes on said body for taking off electrical signals andfor connection to the amplifier, said body having a substantiallyconstant cross section perpendicular to the compressive force,

I said body polarized in a direction d in at least a subvolume (d I 11)thereof where d is the thickness dimension, 1 is the length dimension,and I1 is the height dimension,

said electrodes mounted on opposite surfaces of said body spaced apartthe thickness d,

the thickness d perpendicular to the direction of the compressive force,the length I being perpendicular to the thickness d and to the directionof the compressive force and is greater than the thickness 11,

the height h is perpendicular to the thickness d and the length I and isgreater than 10d, and

a housing mounting said body including means receiving a compressiveforce in one direction and converting the force and applying it to saidbody in a direction perpendicular to said one direction.

32. A key for controlling an amplifier of an electronic circuit inresponse to the application thereto of an intentionally applied anddimensioned compressive force, comprising:

a piezoelectric transducer including a body of piezoceramic material forreceiving the compressive force,

a pair of electrodes on said body for taking off electrical signals andfor connection to the amplifier, said body having a substantiallyconstant cross section perpendicular to the compressive force,

said body polarized in a direction d in at least a subvolume (d I Ii)thereof where d is the thickness dimension, I is the length dimension,and I1 is the height dimension, 6

said electrodes mounted on opposite surfaces of said body spaced apartthe thickness d,-

the thickness d perpendicular to the direction of the compressive force,the length I being perpendicular to the thickness d and to the directionof the compressive force and is greater than the thickness (1,

the height I2 is perpendicular to the thickness d and the length I andis greater than 10d, and

a second pair of electrodes mounted adjacent respective ones of thefirst-mentioned electrodes to form a capacitor for connection in circuitwith the amplifier.

33. A key for controlling an amplifier of an electronic circuit inresponse to the application thereto of an intentionally applied anddimensioned compressive force, comprising:

a piezoelectric transducer including a body of piezoceramic material forreceiving the compressive force,

a pair 'of electrodes on said body for taking off electrical signals andfor connection to the amplifier,

said body having a substantially constant cross section perpendicular tothe compressive force,

said body polarized in a direction d in at least a subvolume (d I 11)thereof where dis the thickness dimension, I is the length dimension,and h is the height dimension,

said electrodes mounted on opposite surfaces of said body spaced apartthe thickness d,

the thickness d perpendicular to the direction of the compressive force,the length I being perpendicular to the thickness d and to the directionof the compressive force and is greater than the thickness d,

the height h is perpendicular to the thickness d and the length I and isgreater than 10d, and

said body having a total height H and including a piezoelectric portionh and a non-piezoelectric portion H I1.

34. The key of claim 33, wherein said piezoelectric portion and saidnonpiezoelectric portion are a single piece, said piezoelectric portionbeing the polarized sub-volume.

35. The key of claim 34, comprising an actuating member and wherein saidnonpiezoelectric portion 'l-L/i is mounted facing said actuating member.

36. An electronic key circuit operable in response to the applicationthereto of an intentionally applied and dimensioned compressive force,comprising:

an amplifier,

a piezoelectric transducer including a body of piezoceramic material forreceiving the compressive force,

a pair of electrodes on said body for taking off electrical signals andconnected to the input of the amplifier,

said body having a substantially constant cross section perpendicular tothe compressive force,

said body polarized in a direction d in at least a subvolume (a' I 11)thereof where d is the thickness dimension. 1 is the length dimension,and h is the height dimension,

said electrodes mounted on opposite surfaces of said body spaced apartthe thickness d,

the thickness a perpendicular to the direction of the compressive force,the length 1 being perpendicular to the thickness d and to the directionof the compressive force and is greater than the thickness d,

the height h is perpendicular to the thickness d and the length I and isgreater than 10d. and

an RC filter. said body having a total height H, a second pair ofelectrodes mounted on opposite surfaces of said body in the zone H-li toform a capacitance of said RC filter.

37. The electronic key circuit of claim 36, wherein said RC filter is alow pass RC filter which comprises a resistor, said resistor defined bya strip of resistance material connecting a first-mentioned electrodeand a second electrode which are mounted on the same surface of saidbody.

38. The electronic key circuit of claim 36, comprising a low pass RCfilter connecting said body and said amplifier and operable to shortcircuit frequencies higher than 100 Hz.

39. The electronic key circuit of claim 38. wherein one of thefirst-mentioned electrodes and the second electrode which is mountedtherewith on the same body surface are electrically connected.

40. The electronic key circuit of claim 38. comprising a high pass RCfilter connected to said body to prevent frequencies of higher than 10Hz from being dischargedi 41. The electronic key circuit of claim 40,wherein said electrodes constitute the capacitor plates of thecapacitance of high pass RC filter and said high pass RC filter includesa resistor connected across the input of said amplifier.

1. A key for controlling an amplifier of an electronic circuit inresponse to the application thereto of an intentionally applied anddimensioned compressive force, comprising: a piezoelectric transducerincluding a body of piezoceramic material for receiving the compressiveforce, a pair of electrodes on said body for taking off electricalsignals and for connection to the amplifier, said body having asubstantially constant cross section perpendicular to the compressiveforce, said body polarized in a direction d in at least a sub-volume (d. l . h) thereof where d is the thickness dimension, l is the lengthdimension, and h is the height dimension, said electrodes mounted onopposite surfaces of said body spaced apart the thickness d, thethickness d perpendicular to the direction of the compressive force, thelength l being perpendicular to the thickness d and to the direction ofthe compressive force and is greater than the thickness d, and theheight h is perpendicular to the thickness d and the length l and isgreater than 10d.
 2. The key of claim 1, wherein said body includes abase surface having an area l . d of less than 5 mm2.
 3. The key ofclaim 2, wherein said base surface has an area of < or = 1 mm2.
 4. Thekey of claim 1, wherein the length l is at least 20d.
 5. The key ofclaim 1, wherein the length l is 50d.
 6. The key of claim 1, wherein thelength l is in a range of from 5 to 20 mm.
 7. The key of claim 1,wherein the length l is dimensioned according to the equation l n. g31 .K/U where K is the compressive force acting upon the surface l . d ofsaid body, g31 is the piezoelectric voltage constant of the material ofsaid body, U is the response voltage necessary foR actuating theelectronic circuit and n is a value in a range of from 0.3 to 2.0. 8.The key of claim 1, wherein the thickness d is less than 0.5 mm.
 9. Thekey of claim 1, wherein the thickness d is in a range of from 0.05 to0.15 mm.
 10. The key of claim 1, wherein said body includes surfaceareas defined by the dimensions l . h and said electrodes coverrespective ones of such surface areas.
 11. A key for controlling anamplifier of an electronic circuit in response to the applicationthereto of an intentionally applied and dimensioned compressive force,comprising: a piezoelectric transducer including a body of piezoceramicmaterial for receiving the compressive force, a pair of electrodes onsaid body for taking off electrical signals and for connection to theamplifier, said body having a substantially constant cross sectionperpendicular to the compressive force, said body polarized in adirection d is at least a sub-volume (d . l . h) thereof where d is thethickness dimension, l is the length dimension, and h is the heightdimension, said electrodes mounted on opposite surfaces of said bodyspaced apart the thickness d, the thickness d perpendicular to thedirection of the compressive force, the length l being perpendicular tothe thickness d and to the direction of the compressive force and isgreater than the thickness d, the height h is perpendicular to thethickness d and the length l and is greater than 10d, and support meansmounted against said body and co-operable therewith to resist bending inresponse to the application of the intentionally applied compressiveforce.
 12. The key of claim 11, wherein said support means, in thedirection of the force, has an integral pressure elastic resiliencewhich is substantially the same as to greater than that of said body.13. The key of claim 11, wherein said support means comprises a pair ofplates fixed to opposite surfaces of said body.
 14. The key of claim 11,wherein said support means comprises a member embedding said body andhaving an X-shaped profile in the direction parallel to the direction offorce application.
 15. The key of claim 13, wherein each of said plateshas dimensions corresponding to the corresponding dimensions of therespective surface on which it is fixed.
 16. The key of claim 15,wherein each of said plates has a thickness which provides the bendingresistance and an integral pressure elastic resilience greater than thatof said body in the direction of the applied force.
 17. The key of claim13, wherein said plates are mounted on the two largest lateral surfacesof said body.
 18. The key of claim 13, wherein said plates are bonded tosaid body.
 19. The key of claim 13, wherein said plates consist ofsynthetic material.
 20. The key of claim 11, wherein said housingincludes a force transmission member, mounting said body only in a smallzone of said body.
 21. The key of claim 20, wherein said housingincludes external surfaces, a force receiving member, and mountingflanges extending from said surfaces adjacent said member.
 22. The keyof claim 11, comprising supporting means including a pair of supportplates, and thermoplastic bonding means securing said plates to oppositesurfaces of said body.
 23. The key of claim 22, wherein said platesinclude recesses for receiving respective electrical connections forsaid electrodes.
 24. The key of claim 23, comprising conductive stripscarried in said recesses and connected to the electrodes.
 25. A key forcontrolling an amplifier of an electronic circuit in response to theapplication thereto of an intentionally applied and dimensionedcompressive force, comprising: a piezoelectric transducer including abody of piezoceramic material for receiving the compressive force, apair of electrodes on said bodY for taking off electrical signals andfor connection to the amplifier, said body having a substantiallyconstant cross section perpendicular to the compressive force, said bodypolarized in a direction d in at least a sub-volume (d.l.h) thereofwhere d is the thickness dimension, l is the length dimension, and h isthe height dimension, said electrodes mounted on opposite surfaces ofsaid body spaced apart the thickness d, the thickness d perpendicular tothe direction of the compressive force, the length l being perpendicularto the thickness d and to the direction of the compressive force and isgreater than the thickness d, the height h is perpendicular to thethickness d and the length l and is greater than 10d, and heatinsulating means surrounding said body.
 26. The key of claim 25, whereinsaid heat insulating means comprises a material which is lesscompression resistant than said body.
 27. A key for controlling anamplifier of an electronic circuit in response to the applicationthereto of an intentionally applied and dimensioned compressive force,comprising: a piezoelectric transducer including a body of piezoceramicmaterial for receiving the compressive force, a pair of electrodes onsaid body for taking off electrical signals and for connection to theamplifier, said body having a substantially constant cross sectionperpendicular to the compressive force, said body polarized in adirection d in at least a sub-volume (d . l . h) thereof where d is thethickness dimension, l is the length dimension, and h is the heightdimension, said electrodes mounted on opposite surfaces of said bodyspaced apart the thickness d, the thickness d perpendicular to thedirection of the compressive force, the length l being perpendicular tothe thickness d and to the direction of the compressive force and isgreater than the thickness d, the height h is perpendicular to thethickness d and the length l and is greater than 10d, and a housing forsaid transducer, said housing including a base, a cover to receive theforce, a body mounted between said base and said cover, and wallsadjoining said base and said cover and having less compressionresistance to the force than to said body.
 28. The key of claim 27,wherein said housing includes a base having a groove therein and a coverhaving grooved means, said body mounted in said groove and said groovedmeans.
 29. A key for controlling an amplifier of an electronic circuitin response to the application thereto of an intentionally applied anddimensioned compressive force, comprising: a piezoelectric transducerincluding a body of piezoceramic material for receiving the compressiveforce, a pair of electrodes on said body for taking off electricalsignals and for connection to the amplifier, said body having asubstantially constant cross section perpendicular to the compressiveforce, said body polarized in a direction d in at least a sub-volume (d. l . h) thereof where d is the thickness dimension, l is the lengthdimension, and h is the height dimension, said electrodes mounted onopposite surfaces of said body spaced apart the thickness d, thethickness d perpendicular to the direction of the compressive force, thelength l being perpendicular to the thickness d and the length l and isgreater than 10d, and a housing for said transducer including a base anda cover plate mounting said body therebetween, and a plurality of sidewalls joining said base and said cover, said side walls being resilientwith respect to the compressive force and said base being slightlyresistant to bending.
 30. A key for controlling an amplifier of anelectronic circuit in response to the application thereto of aNintentionally applied and dimensioned compressive force, comprising: apiezoelectric transducer including a body of piezoceramic material forreceiving the compressive force, a pair of electrodes on said body fortaking off electrical signals and for connection to the amplifier, saidbody having a substantially constant cross section perpendicular to thecompressive force, said body polarized in a direction d in at least asub-volume (d . l . h) thereof where d is the thickness dimension, l isthe length dimension, and h is the height dimension, said electrodesmounted on opposite surfaces of said body spaced apart the thickness d,the thickness d perpendicular to the direction of the compressive force,the length l being perpendicular to the thickness d and to the directionof the compressive force and is greater than the thickness d, the heighth is perpendicular to the thickness d and the length l and is greaterthan 10d, and a second electrode mounted adjacent and electricallyinsulated from and having the same surface as one of the first-mentionedelectrodes, said body being polarized in opposite directions in thezones adjacent said adjacent electrodes.
 31. A eky for controlling anamplifier of an electronic circuit in response to the applicationthereto of an intentionally applied and dimensioned compressive force,comprising: a piezoelectric transducer including a body of piezoceramicmaterial for receiving the compressive force, a pair of electrodes onsaid body for taking off electrical signals and for connection to theamplifier, said body having a substantially constant cross sectionperpendicular to the compressive force, said body polarized in adirection d in at least a sub-volume (d . l . h) thereof where d is thethickness dimension, l is the length dimension, and h is the heightdimension, said electrodes mounted on opposite surfaces of said bodyspaced apart the thickness d, the thickness d perpendicular to thedirection of the compressive force, the length l being perpendicular tothe thickness d and to the direction of the compressive force and isgreater than the thickness d, the height h is perpendicular to thethickness d and the length l and is greater than 10d, and a housingmounting said body including means receiving a compressive force in onedirection and converting the force and applying it to said body in adirection perpendicular to said one direction.
 32. A key for controllingan amplifier of an electronic circuit in response to the applicationthereto of an intentionally applied and dimensioned compressive force,comprising: a piezoelectric transducer including a body of piezoceramicmaterial for receiving the compressive force, a pair of electrodes onsaid body for taking off electrical signals and for connection to theamplifier, said body having a substantially constant cross sectionperpendicular to the compressive force, said body polarized in adirection d in at least a sub-volume (d . l . h) thereof where d is thethickness dimension, l is the length dimension, and h is the heightdimension, said electrodes mounted on opposite surfaces of said bodyspaced apart the thickness d, the thickness d perpendicular to thedirection of the compressive force, the length l being perpendicular tothe thickness d and to the direction of the compressive force and isgreater than the thickness d, the height h is perpendicular to thethickness d and the length l and is greater than 10d, and a second pairof electrodes mounted adjacent respective ones of the first-mentionedelectrodes to form a capacitor for connection in circuit with theamplifier.
 33. A key for controlling an amplifier Of an electroniccircuit in response to the application thereto of an intentionallyapplied and dimensioned compressive force, comprising: a piezoelectrictransducer including a body of piezoceramic material for receiving thecompressive force, a pair of electrodes on said body for taking offelectrical signals and for connection to the amplifier, said body havinga substantially constant cross section perpendicular to the compressiveforce, said body polarized in a direction d in at least a sub-volume (d. l . h) thereof where d is the thickness dimension, l is the lengthdimension, and h is the height dimension, said electrodes mounted onopposite surfaces of said body spaced apart the thickness d, thethickness d perpendicular to the direction of the compressive force, thelength l being perpendicular to the thickness d and to the direction ofthe compressive force and is greater than the thickness d, the height his perpendicular to the thickness d and the length l and is greater than10d, and said body having a total height H and including a piezoelectricportion h and a non-piezoelectric portion H - h.
 34. The key of claim33, wherein said piezoelectric portion and said nonpiezoelectric portionare a single piece, said piezoelectric portion being the polarizedsub-volume.
 35. The key of claim 34, comprising an actuating member andwherein said nonpiezoelectric portion H-h is mounted facing saidactuating member.
 36. An electronic key circuit operable in response tothe application thereto of an intentionally applied and dimensionedcompressive force, comprising: an amplifier, a piezoelectric transducerincluding a body of piezoceramic material for receiving the compressiveforce, a pair of electrodes on said body for taking off electricalsignals and connected to the input of the amplifier, said body having asubstantially constant cross section perpendicular to the compressiveforce, said body polarized in a direction d in at least a sub-volume (d. l . h) thereof where d is the thickness dimension, l is the lengthdimension, and h is the height dimension, said electrodes mounted onopposite surfaces of said body spaced apart the thickness d, thethickness d perpendicular to the direction of the compressive force, thelength l being perpendicular to the thickness d and to the direction ofthe compressive force and is greater than the thickness d, the height his perpendicular to the thickness d and the length l and is greater than10d, and an RC filter, said body having a total height H, a second pairof electrodes mounted on opposite surfaces of said body in the zone H-hto form a capacitance of said RC filter.
 37. The electronic key circuitof claim 36, wherein said RC filter is a low pass RC filter whichcomprises a resistor, said resistor defined by a strip of resistancematerial connecting a first-mentioned electrode and a second electrodewhich are mounted on the same surface of said body.
 38. The electronickey circuit of claim 36, comprising a low pass RC filter connecting saidbody and said amplifier and operable to short circuit frequencies higherthan 100 Hz.
 39. The electronic key circuit of claim 38, wherein one ofthe first-mentioned electrodes and the second electrode which is mountedtherewith on the same body surface are electrically connected.
 40. Theelectronic key circuit of claim 38, comprising a high pass RC filterconnected to said body to prevent frequencies of higher than 10 Hz frombeing discharged.
 41. The electronic key circuit of claim 40, whereinsaid electrodes constitute the capacitor plates of the capacitance ofhigh pass RC filter and said high pass RC filter includes a resistorconnected across the inPut of said amplifier.